1. Field of the Invention
The present invention relates to an optical disk apparatus for recording onto optical disks such as CD-R (compact disk recordable) and DVD-R (digital versatile disk recordable), and, in particular, to sampling and holding of a photoelectrically converted signal in an optical pickup.
2. Description of the Related Art
A photoelectrically converted signal, which is obtained by a built-in photo-detector circuit in an optical pickup and which corresponds to a semiconductor laser beam reflected by a disk at recording time, has the waveform shown in FIG. 5A. The photoelectrically converted signal is normally transmitted to an RF signal processing circuit through a flexible cable. This is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 11-53735 (page 5, FIG. 1). The RF signal processing circuit calculates signals, such as a servo signal and an APC (automatic emitting-power control circuit) control signal, based on the photoelectrically converted signal input through the flexible cable. To calculate the servo signal, the photoelectrically converted signal must be sampled in periods (the low level periods shown in FIG. 5A) in which the semiconductor laser beam is emitted at a bias power. An increase in recording speed shortens the sampling period. Thus, a signal settling time, which is obtained when a photoelectrically converted signal in the recording period changes to the photoelectrically converted signal in bias periods (the low level periods shown in FIG. 5A), must be reduced.
The above flexible cable electrically acts as a distributed parameter line and has a frequency characteristic having a peak in the vicinity of 150 MHz. Therefore, if the slew rate of an output from the photo-detector circuit is enhanced in order to shorten the settling time, as FIG. 5B shows, ringing 50, which occurs at a bias power portion of the photoelectrically converted signal in the bias period (low level period), only increases. Conversely, the settling time lengthens.
In addition, since enhancing the slew rate represents short-time charging for a capacitive element in the flexible cable and a load capacitance such as an input capacitance of the RF signal processing circuit at a post stage, a circuit current flowing at a final stage of the photo-detector circuit must be increased. This increases the power consumption, and the increased power often exceeds the allowable value in a package. Accordingly, also in this sense, the settling time cannot be shortened. Therefore, it is very difficult to shorten the settling time from the recording power level to the bias power level from the order of ten nanoseconds. This results in the inability to cope with increased recording speed, thus decreasing the servo accuracy. In some cases, tracking servo control is impossible.
Accordingly, in one method, it is possible that, by providing a sample-and-hold circuit in the photo-detector circuit, and sampling and holding the photoelectrically converted signal before performing transmission through the flexible cable, a sampled-and-held signal having a small amplitude is transmitted through the flexible cable. This prevents the signal transmitted through the flexible cable from experiencing a large change in level. Accordingly, the need for a circuit having a large slew rate is eliminated. In addition, a circuit including a hold capacitor whose capacitance is large is not required (up to 11 T of 1×CD, that is, approximately 100 kHz) as the sample-and-hold circuit. Thus, a large current is not required as a driving current in a current-voltage converting circuit at a previous stage. Moreover, since the elimination of the need to drive the flexible cable results in a small load, a slew rate at an initial stage can be sufficiently enhanced, thus shortening a signal settling time.
High speed recording has a high clock frequency. For example, in 16×DVD recording, the clock frequency is approximately 400 MHz. Thus, when signal sampling is performed in a bias power emitting interval (3 T), which is the minimum length, even in the case of performing sampling from 7.5 nanoseconds (4 T), the photoelectrically converted signal must be settled in 10 nanoseconds or less or the sampling cannot be achieved by the present circuit.
Push-pull methods and three-beam methods have conventionally been employed as tracking servo methods for apparatuses for recording on or playing back optical disks. Among these methods, a differential push-pull method is typically used.
The principle of the differential push-pull method is shown in FIG. 6. As shown in FIG. 6, three beam spots (a main spot M and side spots S1 and S2) formed by a diffraction grating are positioned so that the side spots S1 and S2 are disk-radially shifted with respect to the main spot M by half of a track pitch P. Reflected beams from the main spot M and both side spots S1 and S2 are photoelectrically converted by photo-detectors so that push-pull signals can be obtained for the spots M, S1, and S2. The photoelectrically converted signals corresponding to the spots M, S1, and S2 must be sampled in the above-described manner.
Regarding the photoelectrically converted signal corresponding to the main spot M, when considering detection of an address signal, sampling must be performed within a 3 T space, which is the shortest repetition. For the photoelectrically converted signal corresponding to each side spot, only a servo signal needs to be detected. Thus, conventionally, it is preferable to perform signal detection concerning a signal having 6 T or longer. However, there is a problem in that, when the main spot output and each side spot output are sampled with the same sampling timing, a circuit for the side spot must also have a bandwidth and settling characteristic similar to those in a circuit for the main spot. The bandwidth must be extremely broadened, so that the circuit for the side spot does not sufficiently operate. Accordingly, the circuit operation is unstable. For eliminating this defect, design and circuit improvements are required. This causes various problems, such as an increase in circuit size and an increase in production cost. Accordingly, it is preferable that different timing be used for each of the circuit for the main spot M and the circuit for each side spot. However, this complicates a sampling-timing-signal generator, and the number of wires for transmitting sampling-timing signals is increased, thus increasing flexible cable size. This causes a problem in that reduction in apparatus size cannot be achieved.